Apparatus for stabilization treatment of ferromagnetic metal powder

ABSTRACT

An apparatus for stabilizing treatment of ferromagnetic metal powder which apparatus makes it possible to form a surface-oxidized film on the powder which is as uniform and dense as possible and to afford ferromagnetic metal powder having a good quality, and which apparatus is characterized by comprising a horizontal, cylindrical, rotating type body of reactor; a feeding port for the ferromagnetic metal powder into the body; a withdrawing port for the ferromagnetic metal powder from the body; a blowing passageway for an oxidizing gas into the body; and a withdrawing passageway for the oxidizing gas from the body.

This application is a continuation of application Ser. No. 07/154,324,filed Feb. 10, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus for stabilization treatment offerromagnetic metal powder, and more particularly it relates to anapparatus for stabilization treatment of ferromagnetic metal powderparticularly composed mainly of iron.

2. Description of the Related Art

The recent development of magnetic recording is noteworthy and advancesin magnetic powder used therefor has also contributed, to the abovedevelopment. Further, in order to achieve high density magneticrecording, development of ferromagnetic metal powder having a highcoersive force and high saturation magnetization has been advanced.

However, since this ferromagnetic metal powder has a very high chemicalactivity due to its fine particles, the powder, if subjected to notreatment, is at once oxidized upon its exposure to air to lose itsspecific feature as its intrinsic magnetic material. In addition to suchproblem, heat generation and ignition occur due to its sudden oxidationto raise a problem of its handling safety. Still another problem israised with respect to corrosion resistance which is most importantamong practical specific features of magnetic tape (hereinafterabbreviated merely to "tape").

Thus, in order to assure the quality and handling safety offerromagnetic metal powder and the corrosion resistance of tape,adhesion of organic substances onto the surface of ferromagnetic metalpowder or the surface oxidation treatment of ferromagnetic metal powderhave been carried out.

However, as to the surface treatment with organic substances, a certaineffect upon the stability of the magnetic powder itself is observed, butat the time of coating of the resulting powder, the number of choices ofsurfactant, solvent, etc. used for dispersing the organic substances islimited, thereby making difficult dispersion of coating more difficult;hence a high quality tape not only could not have been obtained, butalso the treatment has been uttely ineffective to provide corrosionresistance to a tape.

As compared with the above-mentioned process, the surface oxidationtreatment process i.e. the oxidized coating-forming process usingdiluted oxygen as an oxidizing gas has currently been most broadlyemployed and is an effective process. As an apparatus for practicing theprocess, reactors of a stirring vessel type (Japanese patent applicationlaid-open No. Sho 55-164001/1980), a fluidizing vessel type (U.S. Pat.No. 4,420,330), and a fixed vessel type (Japanese patent applicationlaid-open No. Sho 57-19301/1982) have been known, but a problem has beenraised with respect to the apparatus used for forming a uniform anddense oxidized coating to thereby produce powder having a good quality.

The reaction which proceeds by means of the reactor of stirring vesseltype is directed to a process of dispersing ferromagnetic metal powderin an organic solvent with stirring and blowing an oxidizing gas intothe resulting dispersion to thereby form an oxidized film on theferromagnetic metal powder, but in order to uniformly disperse theferromagnetic metal powder in the organic solvent, it is inevitablynecessary to grind powder granules into primary particles or secondaryparticles. However, the surface energy of ferromagnetic metal powder isso large that when the powder is ground, an agglomerate occurs; hencethe surface oxidation not only becomes uneven, but also the agglomeratedoes not disintegrate to smaller particle so that it has a bad influenceupon the physical properties of tape. Further, since it is necessary tocarry out agitation inside the reaction vessel, a stirrer is providedtherein; thus this agitation causes collision of particles with oneanother, resulting in pulverization and further agglomeration to therebydeteriorate the physical properties.

The reactor of the fluidizing vessel type is directed to an apparatuswherein ferromagnetic metal powder is fluidized in a gas to form anoxidized film, but it is difficult to establish fluidizing conditionsand it is necessary to carry out good fluidization for uniform coating;thus pulverization and agglomeration due to collision of particles withone another occur so that a non-uniform oxidized film is not onlyformed, but also the physical properties of ferromagnetic metal powderare deteriorated.

As to the reactor of the fixed vessel type, since ferromagnetic metalpowder is not moved, pulverization due to collision of particles withone another does not occur, but non-uniform oxidation due to uneven flowof gas occurs and in an extreme case, reaction abruptly proceeds due tolocal oxidation heat-generation; thus there is a fear that the operationitself is impossible.

As described above, conventional apparatus has been insufficient as anapparatus for producing ferromagnetic metal powder having a good qualityand a stabilized surface.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an industrialapparatus for stabilizing ferromagnetic metal powder according to thesurface oxidization treatment process, which apparatus makes it possibleto form a surface-oxidized film which is as uniform and dense aspossible and to afford ferromagnetic metal powder having a good quality.

The present invention resides in an apparatus for stabilizing treatmentof ferromagnetic metal powder, which apparatus comprises

a horizontal, cylindrical, rotating type reactor body;

a port for feeding said ferromagnetic metal powder into said body ofreactor;

a port for withdrawing said ferromagnetic metal powder from said reactorbody, which port may be used in common to said feeding port;

a passageway for blowing an oxidizing gas into said body of reactor; and

a passageway for withdrawing said oxidizing gas from said body ofreactor.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1, 2, 4 and 6 each show a cross-sectional schematic viewillustrating an embodiment of the apparatus of the present invention.

FIG. 3 shows a cross-sectional schematic view along the line III--III ofFIG. 2.

FIG. 5 shows a cross-sectional schematic view along the line V--V ofFIG. 4.

FIG. 7 shows a cross-sectional schematic view along the line VII--VII ofFIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the accompanying drawings, numeral 1 shows the body of a reactor; 2,a feeding port for raw material (ferromagnetic metal powder) in commonwith a withdrawing port for ferromagnetic matal powder afterstabilization treatment; 3,4, inlet and exit or outlet for oxidizing gas(either of which may be inlet or outlet; 3, inlet and 4, exit, in FIGS.1, 2, 4 and 6); and 5, a pipe located at the central part of the bodythrough which an oxidizing gas passes. The pipe 5 is supported by apedestal (not shown) on the body of the reactor 1. The numeral 6 shows ashaft supporting the body; 7, a supporting roller; 8, a ground packingand a bearing. A gas or a liquid is sealed by the ground packing and thebody of the reactor or the pipe located at the central part of the bodythrough which an oxidizing gas gasses is supported by the bearing.Numeral 9 shows a box supporting the body of the reactor and 10, a gasduct. The inlet or exit 3 for the oxidizing gas and the pipe 5 locatedat the central part of the body through which the oxidizing gas passesconstitute a passageway for blowing or withdrawing the oxidizing gas.The exit or inlet 4 for the oxidizing gas, the box 9 supporting the bodyof the reactor, the gas duct 10 and ground packings and bearings 8between the box supporting the body of the reactor 9, the gas duct 10and the pipe 5 at the central part of the body through which anoxidizing gas passes, constitute the withdrawing passageway or theblowing-in passageway for the oxidizing gas. Numeral 11 shows a drivingmotor for rotating the body of the reactor; 12, a support for thereaction apparatus; 13, a scraping-up plate; and 14, 15, inlet andoutlet or exit for heating medium or cooling medium (either of which maybe inlet or exit; and in FIGS. 4 and 6, numeral 14 shows inlet and 15,exit); 16, a jacket; 17, a box supporting the body of the reactor; 18,an outer side duct for heating (or cooling), i.e., a heat transfermedium; 19, an inner side duct for heating (or cooling), i.e., a heattransfer medium; and 20, granules of ferromagnetic metal powder. Arrowmarks indicate the flow direction of the oxidizing gas, the flowdirection of heating or cooling medium and the rotating direction of thebody of the reactor.

A raw material composed mainly of iron and also containing at least onecomponent of metal elements other than iron such as Ni, Si, Al, Mn, Cu,Cr, Ti, Mg, Co, Zn, Ba, Sn, etc. and compounds of these metal elements,the content of the metal element being in the range of 0 to 50% byweight based on the weight of iron, is molded into granules having aminimum diameter of 0.25 to 10 mm, and it is preferred to use suchgranules. If the proportion of the metal elements other than iron andcompounds thereof exceeds 50% by weight based on the weight of iron,there is a fear that the ferromagnetic characteristics are lost.Further, if the minimum diameter of granules of ferromagnetic metalpowder is less than 0.25 mm, the granules are scattered by an oxidizinggas to the outside of the body of the reactor 1. If the minimum diameterof the granules exceeds 10 mm, there is a fear that the surfaceoxidation of the ferromagnetic powder becomes non-uniform.

As the oxidizing gas, an inert gas containing oxygen is preferred touse. The oxygen concentration of the oxidizing gas has no particularlimitation, but if the gas is too dilute, the reaction is so retardedthat it is practically undesirable. Thus, 0.5% by volume or more ispreferred. On the other hand, when a solvent is used, the upper limitshould be restricted to its explosion range or less for safety. As tothe dilution of oxygen in the oxidizing gas, use of an inert gas raisesno problem, and nitrogen gas may be advantageously used. Further, thegas quantity is preferred to be adjusted so that 5 to 30 hours may berequired for making the quantity of the surface layer oxidized(hereinafter referred to "conversion") 10 to 25%. If the oxidation rateis too high, there is a fear of deterioration of the physical propertiesdue to local heat generation.

As to the body of the reactor 1, a horizontal, cylindrical, rotatingtype reactor is used, since this horizontal, rotating type reactor canuniformly mix granules of ferroelectric metal powder. In short, thegranules within the layer of the granules ascend with rotation of thebody of the reactor 1, and after the granules reach the upper end of thegranule layer, they drop on the surface of the granule layer. As theydrop, they are exposed to the oxidizing gas. Thus, the moving of thegranules is uniformly carried out. The granules within the granule layerin the reactor are moved and uniformly mixed as described above, wherebyit is possible to form a dense surface-oxidized film on the surface ofthe ferromagnetic metal powder.

Further, another important point of the apparatus of the presentinvention consists in that the apparatus lacks any means such asstirrer, etc., which causes the granules of ferromagnetic metal powderto be mechanically pulverized. When the granules of ferroelectric metalpowder are pulverized, the powder particles are broken, and further,sintering of the powder particles due to contact thereof with oxidizinggas, agglomeration, etc. occurs, causing deterioration of the physicalproperties of the powder particles. In the case of the horizontalreactor of the present invention, the factor of causing pulverization isminimal from contact of the granules of ferromagnetic metal powder onlywith one another; hence the reactor constitutes a structure by whichpulverization hardly occurs so that it is possible to restrain thepulverization which deteriorates the physical properties of finallyobtained, stabilized ferromagnetic metal powder, to a minimum; thus theresulting powder has a good quality.

Oxygen as an oxidizing gas is removed from the inside of the body of thereactor 1 using nitrogen gas or the like in advance of introducing thegranules of ferromagnetic metal powder only into the body. Next, thegranules of ferromagnetic metal powder are introduced into the body ofthe reactor 1 through a feed part or supply valve 2 positioned upwardlest the granules should be contacted with air. The quantity of thegranules introduced is preferred to be 50% or less, preferably 40% orless of the capacity of the body of the reactor 1, in order to obtain auniform mixing effect. The granules of ferromagnetic matal powder may beeither in a state where they are immersed in an organic solvent or in astate where they are not immersed therein, in the body of the reactor 1.In the case where they are immersed therein, the mixed quantity of thegranules of ferromagnetic metal powder and the organic solvent ispreferred to be 40% or less of the capacity of the reactor. Examples ofusable organic solvents are aromatic hydrocarbons such as benzene,toluene, xylene, etc., fluorine compound solvents such astrifluoroethanol, perfluorooctane, etc., and lower alcohols such asmethanol, ethanol, etc.

The granules of ferromagnetic metal powder introduced are uniformlymixed by rotation of the body of the reactor 1 by means of a drivingmotor 9. By uniformly mixing the granules of ferromagnetic metal powder,it is possible to form a dense and uniform oxidized film on the surfaceof the granules through their contact with the oxidizing gas.

In order to further improve the uniformity and denseness of the oxidizedfilm, it is preferred to provide a scraping-up plate 13 on the innerwall of the body of the reactor 1 in parallel to the generatrix thereof.As shown in FIGS. 2, 3, 6 and 7 a plurality of scraping-up plates isprovided. The scraping-up plate yields a scraping-up effect and theamount of the mixture increases and may be concentrated over a smallerportion of the circumference of the inner cylindrical wall of thereactor body, as shown in FIG. 7, to thereby increase the uniformity ofoxidation. Further, since oxidation reaction at the part where themixture is scraped up by means of the scraping-up plate can also becarried out, it is possible to shorten the reaction time. When a solventis used, the reaction in the solvent is heterogeneous and retarded andhence not practical; thus it is necessary to fix a scraping-up plateonto the inner wall, and then advance the reaction while the granulesare scraped up from the solvent by means of the scraping-up plate anddrop into the solvent. Thus, in this case, it is indispensable toprovide the scraping-up plate.

The revolution number of the body of the reactor 1, and the width, thenumber and the angle with respect to the central line of the body of thereactor 1, of the scraping-up plate are optional, but these arepreferred to be determined taking into consideration that the oxidationrate and pulverization due to rotation of ferromagnetic metal powderwhich somewhat occurs due to the fall of the granules from thescraping-up plate, friction of the granules between one another, etc.,is to be made as low as possible and also the granules are to beuniformly dispersed within the body of the reactor 1.

An oxidizing gas for oxidizing the granules of ferromagnetic metalpowder is blown into the body through a port 3 or 4. It is necessary forthe blow-in of the oxidizing gas to uniformly blow the gas toward thegranules of ferromagnetic metal powder being uniformly mixed by therotation of the body of the reactor 1 or the rotation and thescraping-up plate to thereby form a uniform and dense oxidized film. Ifthe oxidizing gas is locally blown toward the granules of ferromagneticmetal powder or oxygen in the oxidizing gas is non-uniform, then theresulting oxidized film becomes non-uniform, however uniformly thegranules are mixed. If the speed of the oxidizing gas is low, the oxygenconcentration in the oxidizing gas in the axial direction within thebody of the reactor 1 varies depending on the oxygen consumption by thereaction and hence is liable to be non-uniform. On the other hand, ifthe blowing speed is high, local heat generation occurs to deterioratethe physical properties of the granules. Thus, the speed of theoxidizing gas blown toward the granules of ferromagnetic metal powder ispreferred to be in the range of 0.01 to 2 m/sec.

As described above, due to the effect of the rotation of the body of thereactor 1 or the rotation of the scraping-up plate 13, the granules offerromagnetic metal powder are uniformly mixed and the oxidizing gas isblown through the port 3 or 4 to form a uniform oxidized film on thesurface of the powder; however, in order to control the reactiontemperature optionally and with safety, it is further preferred toprovide a jacket 16 on the body of the reactor 1 as shown in FIGS. 4, 5,6 and 7. As to the heat generation due to the oxidation, when iron isoxidized to magnetite (Fe₃ O₄), the quantity of heat generated is 1,600Kcal per Kg of iron. Due to the reaction heat, the granules offerromagnetic metal powder generate heat and due to overheating at thattime, there is a fear that the reaction abruptly proceeds resitting indeterioration the physical properties of the granules. Further, even inthe case of slight overheating, the oxidized film on the surface of theferromagnetic metal powder is liable to becomes non-uniform. Thus, inorder to prevent such overheating to thereby form a dense oxidized film,it is preferred to control the reaction temperature. On the other hand,when a solvent is used, if the temperature is low, the reaction does notproceed so that it is necessary to raise the temperature. Thus, it ispreferred to provide a jacket 16 on the body of the reactor 1.

The efficiency of contact of the ferromagnetic metal powder with theoxidizing gas varies depending on the structure and the rotation numberof the body of the reactor 1, the quantity of the raw material fed, thesize of the scraping-up plate 13, the quantity and concentration of theoxidizing gas blown through the port 3 or 4 and use or nonuse of thesolvent, and hence an adequate inner temperature of the reactor (thetemperature referring to the temperature of the granules in the reactor;and this applying to the subsequent); thus it is possible to afford adesired inner temperature by adjusting the jacket temperature. The innertemperature of the reactor is preferred to be generally in the range of0° to 90° C. If the temperature is lower than 0° C., a problem is raisedin that the cooling cost is high and the reaction time is prolonged;hence such low temperature is commercially unadvantageous. On the otherhand, if the temperature is higher than 90° C., the oxidization reactionrapidly proceeds so that the oxidized film cannot be dense to therebydeteriorate the physical properties of the ferromagnetic metal powder.

When an organic solvent is used, the organic solvent vaporizes into theoxidizing gas and is withdrawn from the body of the reactor to theoutside of the system, accompanying the gas. At that time, the solventmay be either withdrawn as it is, or condensed by a condenser, recoveredand returned to the body of the reactor 1. Further, it is also possibleto adequately withdraw the solvent midway through a valve 2 positionedbelow.

The reaction can be adjusted so as to give a conversion of 10 to 25% anda reaction time of 5 to 30 hours, through uniform mixing of the granulesof ferromagnetic metal powder by the rotation of the body of the reactor1 and if desired, rotation and the scraping-up plate 13, uniformoxidation by blowing the oxidizing gas therein through the port 3 or 4,and if desired, control of the inner temperature by adusting thetemperature of the jacket 16; thus it is possible to form a uniform anddense oxidized film on the ferromagnetic metal powder.

The resulting granules of ferromagnetic metal powder having a uniformand dense oxidized film formed thereon are withdrawn through the valve2, and as to the withdrawing manner, the body 1 is tilted and rotatedand the granules are collected to the side of the valve 2 and withdrawnthrough the valve 2 positioned below.

As described above, according to the present invention, the granules offerromagnetic metal powder are not only uniformly mixed due to theeffect of the rotation of the body of the reactor or the rotation of thescraping-up plate, but also almost no pulverization having a badinfluence upon the physical properties of the ferromagnetic metal powderoccurs; hence a uniform and dense oxidized film is formed on the surfaceof the powder so that it is possible to obtain granules of ferromagnetic metal powder having a good quality and a stabilized surface.

The effectiveness of the present invention will be described im moredetail by way of reference examples and comparative examples.

REFERENCE EXAMPLE 1

Into the body of a reactor 1 in a stabilization apparatus having itsinside purged with nitrogen gas in advance, as shown in FIGS. 6 and 7,which body has a length of 1,200 mm and a diameter of 400 mm and isprovided with 4 scrapin-up plates 13 each having a width of 75 mm, werefed granules of ferro magnetic metal powder of 1 to 2 mm in diameter and2 to 5 mm long (10 Kg on the basis of dry weight) through a valve 2,which granules were immersed in toluene (35 Kg) and which ferromagneticmetal powder contained 10% by weight of Ni, 1.5% by weight of Al, 2.5%by weight of Si, each weight based on the weight of iron and theremainder being iron. A motor 11 was driven and the body of the reactor1 was rotated at 2 rpm. Warm water was passed through a jacket 16 toraise the temperature of the jacket up to 40° C. When 40° C. wasattained, nitrogen gas containing 5% by volume of oxygen was blown intothe body through a gas inlet 3 at a rate of 30 Nm³ /Hr. In such a state,reaction was carried out for 15 hours. During the period, toluenedistilled out accompanying the oxidizing gas was condensed by acondenser fixed onto the outer part and returned to the inside of theapparatus. After the reaction for 15 hours, while the reactionconditions were maintained, the toluene distilled out accompanying theoxidizing gas was withdrawn to the outside of the reaction system anddrying of the granules of ferromagnetic metal powder was started. Tenhours were required until toluene in the apparatus was entirely freed.At this point, the reaction was stopped and the resulting treatedgranules of ferromagnetic metal powder were withdrawn through a valve 2positioned downward.

The physical properties and conversions of the ferromagnetic metalpowder forming the granules prior to the treatment and after treatmentare shown in Table 1. In additon, the conversion was sought regardingthe iron as having been converted into magnetite and also regarding thismagnetite as having no magnetic properties (this applies to thesubsequent conversion).

REFERENCE EXAMPLE 2

Into the same apparatus as used in Reference example 1 was fed the samegranules of ferromagnetic metal powder as used in Reference example 1 inthe same quantity and immersed in toluene (35 Kg). In the same manner asin Reference example 1, the jacket temperature was made 40° C., nitrogengas containing 5% by volume of oxygen was blown into the body at a rateof 30 Nm³ /Hr and the body of the reactor was rotated at 2rpm. Undersuch conditions, reaction was carried out for 15 hours. During theperiod, toluene distilled out accompanying the oxidizing gas wasrecovered and returned to the apparatus. After the reaction for 15hours, the rotation of the body of the reactor was stopped, and tolueneinside the apparatus was withdrawn through a valve 2 positioned downwardand provided with a wire gauze. However, toluene adhering to the innerwall of the body of the reactor and the granules of ferromagnetic metalpowder remained inside the body of the reactor. Thereafter, the rotationof the body of the reactor was again carried out at 2 rpm. The samereaction conditions such as the jacket temperature, the quantity of thegas blown, etc. as above were employed. The distilled-out toluene waswithdrawn to the outside of the system and the granules of ferromagneticmetal powder inside the apparatus were dried over 8 hours. After thedruing, the granules of ferromagnetic metal powder were withdrawnthrough the valve 2.

The physical properties and conversions of the ferromagnetic metalpowder forming the granules prior to the treatment and after treatmentare shown in Table 1.

REFERENCE EXAMPLE 3

The same apparatus and granules of ferromagnetic metal powder as inReference example 1 were used and reaction was carried out under thesame conditions as in Reference example 1 except that distilled-outtoluene was not returned and all withdrawn to the outside of the systemand when the granules of ferromagnetic metal powder were dried (10 hoursince the start of the reaction), the granules were withdrawn.

The physical properties and conversions of the ferromagnetic metalpowder forming the granules prior to the treatment and after treatmentare shown in Table 1.

REFERENCE EXAMPLE 4

The same apparatus and granules of ferromagnetic metal powder as inReference example 1 were used, and reaction was carried out under thesame conditions as in Reference example 1 except that the jackettemperature was made 60° C. After reaction for 15 hours, the granules offerromagentic metal powder were dried over 7 hours in the same manner asin Reference example 1 except that the jacket temperature was made 60°C., and were withdrawn.

The physical properties and conversions of the ferromagnetic metalpowder forming the granules prior to the treatment and after treatmentare shown in Table 1.

REFERENCE EXAMPLE 5

Using the same apparatus and granules of ferromagnetic metal powder andunder the same conditions as in Reference example 4, reaction wascarried out for 15 hours. After 15 hours, toluene was withdrawn in thesame manner as in Reference example 2 and drying was then carried outunder the initial reaction conditions. After the granules offerromagnetic metal powder were dried for 5 hours, they were withdrawn.

The physical properties and conversions of the ferromagnetic metalpowder forming the granules prior to the treatment and after treatmentare shown in Table 1.

REFERENCE EXAMPLE 6

Reaction was carried out using the same apparatus and under the sameconditions as in Reference example 4 except that distilled-out toluenewas not returned and was withdrawn to the outside of the system. Afterthe granules of ferromagnetic metal powder were dried for 7 hours, theywere withdrawn through the valve 2.

The physical properties and conversions of the ferromagnetic metalpowder prior to the treatment and after treatment are shown in Table 1.

REFERENCE EXAMPLE 7

Into the same apparatus as in Reference example 1 were fed the samegranules of ferromagnetic metal powder in the same quantity togetherwith toluene in the same quantity each as in Reference example 1. Next,toluene was withdrawn through the valve 2 positioned downward and havinga wire gauze attached thereto, in advance of the reaction. However,toluene adhered to the inner wall of the body of the reactor andgranules of ferromagnetic metal powder remained inside the body of thereactor. The body of the reactor was rotated at 2 rpm. N₂ gas containing1% by volume of oxygen was blown into the body through a gas inlet 3.Reaction was continued until the granules of ferromagnetic metal powderwere dried. Seventeen hours were required. The granules were thenwithdrawn.

The physical properties and conversions of the ferromagnetic metalpowder prior to the treatment and after treatment are shown in Table 1.

REFERENCE EXAMPLE 8

Reaction was carried out using the same apparatus and granules and underthe same conditions each as in Reference example 7 except that thejacket temperature was made 20° C. However, the reaction was carried outuntil the granules were dried. Fifteen hours were required.

The physical properties and conversions of the ferromagnetic metalpowder forming the granules prior to the treatment and after treatmentare shown in Table 1.

REFERENCE EXAMPLE 9

The same granules of ferromagnetic metal powder (10 Kg) as in Referenceexample 1 was fed as they were, without being immersed in toluene, intothe same apparatus as in Reference example 1, and purged with nitrogengas, through the valve 2. The body of the reactor was rotated by meansof a driving motor 11 at 2 rpm. Water was passed through the jacket 16to make the jacket temperature 20° C. Nitrogen gas containing 0.5% ofoxygen was blown into the body through the inlet 3 at a rate of 30 Nm³/Hr and reaction was carried out for 20 hours. The body of the reactorwas then tilted and the granules of the surface-treated ferromagneticmetal powder were withdrawn through the valve 2.

The physical properties and conversions of the ferromagnetic metalpowder prior to the treatment and after treatment are shown in Table 1.

All of the treated granules of ferromagnetic metal powder were unchangedin state from those prior to the treatment and free-flowing and therewas almost no pulverization.

COMPARATIVE EXAMPLE 1

the same granules of ferromagnetic metal powder (10 kg) as in Referenceexample 1 were ground by means of a mill in toluene into particles ofabout 0.05 mm in diameter, and the particles were fed into a vertical,cylindrical reactor of 400 mm in diameter and 1,000 mm high equippedwith a jacket and having a stirrer. The height of the particle layer was160 mm. Toluene was additionally added up to a height of 800 mm in thereactor. While the stirrer was rotated at 180 rpm and the particles weredispersed, the jacket temperature was raised up to 60° C. and N₂ gascontaining 5% by volume of oxygen was blown into the reactor at a rateof 30 Nm³ /Hr to carry out oxidation. At that time, toluene accompanyingthe gas was condensed by a condenser and returned to the reactor. Afterthe reaction for 20 hours, the gas feed was stopped and the jackettemperature was lowered down to 20° C. The stirrer was also stopped.Particles after the surface oxidation-treatment were filtered off andtoluene was vaporized at room temperature in N₂ current to dry theparticles, followed by measuring the physical properties and conversionof the particles. The results are shown in Table 1. As compared withReference example 1, it is evident that notable deterioration in thephysical properties occurred.

COMPARTIVE EXAMPLE 2

The same granules of ferromagnetic metal powder (10 Kg) as in Referenceexample 1 were fed as they were, without being immersed in toluene, intoa fixed bed reactor of 400 mm in diameter and 800 mm high equipped witha jacket and designed so as to blow an oxidizing gas from its lower partthereinto. The reactor was purged with nitrogen gas in advance. Theheight of the layer of the granules was 20 cm. The jacket temperaturewas made 20° C. and nitrogen gas containing 0.5% by volume of oxygen wasblown into the reactor from its lower part at a rate of 10 Nm³ /Hr tocarry out reaction for 30 hours. After the reaction, the resultinggranules of the surface-treated ferromagnetic metal powder werewithdrawn from the reactor into the air. As a result, the granulesimmediately became red hot.

                  TABLE 1                                                         ______________________________________                                                         Relative Saturation                                                    Coersive                                                                             square-  magnetiza-                                                                              Conver-                                             force  ness     tion      sion.sup.*(1)                                       [Oe]   [-]      [emu/g]   [%]                                       ______________________________________                                        Physical properties                                                                       1510     0.51     165      0                                      before treatment                                                              Ref. ex. 1  1520     0.51     125     19                                      Ref. ex. 2  1520     0.51     128     17                                      Ref. ex. 3  1520     0.51     135     14                                      Ref. ex. 4  1515     0.51     120     21                                      Ref. ex. 5  1515     0.51     123     20                                      Ref. ex. 6  1515     0.51     132     15                                      Ref. ex. 7  1520     0.51     143     10                                      Ref. ex. 8  1520     0.51     132     15                                      Ref. ex. 9  1500     0.51     120     21                                      Comp. ex. 1 1350     0.46     125     19                                      Comp. ex. 2 Measurement was impossible due to ignition.                       ______________________________________                                         .sup.*(1) The conversion was sought regarding the iron on the surface of      the ferromagnetic metal powder as being oxidized into magnetite (Fe.sub.3     O.sub.4) and also regarding the magnetite as having no magnetic               properties.                                                              

What we claim is:
 1. An apparatus for stabilizing treatment offerromagnetic metal powder, which apparatus comprises:a horizontal,cylindrical, rotating type reactor body provided with end wallssubstantially perpendicular to a cylindrical side wall which forms aninner wall of said reactor body; a jacket for a heat transfer medium; aport including a valve for feeding said ferromagnetic metal powder intosaid reactor body; a port for withdrawing said ferromagnetic metalpowder from said reactor body, which port may be used in common withsaid feeding port; a passageway for blowing an oxidizing gas into saidreactor body comprising an oxidizing gas inlet in communication with apipe arranged within said reactor body coaxially with respect to theaxis of rotation of said reactor body, said passageway arranged to be insubstantial non-contacting relationship with ferromagnetic metal powderand said pipe, having an outlet end, extends longitudinally through saidreactor body, said outlet end positioned proximate the side wall remotefrom said oxidizing gas inlet; a passageway for withdrawing saidoxidizing gas from said reactor body comprising an oxidizing gas outletin communication with a duct connected to said reactor body and arrangedcoaxially with respect to the axis of rotation of said reactor body,said passageway arranged to be in substantial non-contactingrelationship with ferromagnetic metal powder; and a plurality ofscraping up plates for said ferromagnetic metal powder arranged on saidinner wall of said reactor body parallel to the axis of rotation of saidreactor body and extending longitudinally from one end of the reactorbody to the other end thereof.
 2. An apparatus according to calim 1wherein said plurality of scraping-up plates comprises two scraping-upplates.
 3. An apparatus for stabilizing treatment of ferromagnetic metalpowder, which apparatus comprises:a horizontal, cylindrical, rotatingtype reactor body provided with end walls substantially perpendicular toa cylindrical side wall which forms an inner wall of said reactor body;a jacket for a heat transfer medium; a port including a valve forfeeding said ferromagnetic metal powder into said reactor body; a portfor withdrawing said ferromagnetic metal powder from said reactor body,which port may be used in common with said feeding port; a passagewayfor blowing an oxidizing gas into said reactor body comprising anoxidizing gas inlet in communication with a duct connected to saidreactor body and arranged coaxially with respect to the axis of rotationof said reactor body, said passageway arranged to be in substantialnon-contacting relationship with ferromagnetic metal powder; apassageway for withdrawing said oxidizing gas from said reactor bodycomprising an oxidizing gas outlet in communication with a pipe arrangedwithin said reactor body coaxially with respect to the axis of rotationof said reactor body, said passageway arranged to be in substantialnon-contacting relationship with ferromagnetic metal powder and saidpipe, having an inlet end, extends longitudinally through said reactorbody, said inlet end positioned proximate the side wall remote from saidoxidizing gas outlet; and a plurality of scraping up plates for saidferomagnetic metal powder arranged on said inner wall of said reactorbody parallel to the axis of rotation of said reactor body and extendinglongitudinally from one end of the reactor body to the other endthereof.
 4. An apparatus according to claim 3 wherein said plurality ofscraping-up plates comprises two scraping-up plates.